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Author Topic: [SOLVED] 3.3v Regulator drops to 2.8v under load - capacitor not big enough?  (Read 1043 times)
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Hi all,

The basics of my circuit: Input is 10VDC (at 1A) from a plug pack, I have several regulators putting out 5V, 6V and 3.3V, and they all have 0.1uF capacitors across the outputs. Currently they're all working fine except for the 3.3v one.

I have attached to the 3.3v regulator a laser diode which I have set to draw 280mA (it has its own constant current circuit). With the diode removed from the circuit (only the 328 and quiescent current to worry about) it puts out a steady 3.28v, which is close enough for my liking. However, when attaching the diode, it draws 280mA as expected, but the voltage output from the regulator drops to about 2.8v. The regulator is rated at 1.5A, and is also attached to a heat sink that provides adequate cooling (10 minutes operation and it's only warm to the touch).

I have the 0.1uF capacitor attached across the output, but the datasheet's example shows a 10uF capacitor across both the input and output of the regulator. Is the issue here with my capacitor not being big enough? I'm stumped as to what is going wrong here.
« Last Edit: August 01, 2011, 02:28:11 am by montymintypie » Logged

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Unlikely it's related to capacitor, what is the regulator?
And did you actually measure 280 mA at the output?
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Here's the regulator.

And the datasheet (warning: PDF)

And I have measured the 280mA at the output. Granted, before it warms up it's about 170, but it rises to that.
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I have the 0.1uF capacitor attached across the output, but the datasheet's example shows a 10uF capacitor across both the input and output of the regulator. Is the issue here with my capacitor not being big enough? I'm stumped as to what is going wrong here.

The datasheet for the LM1086 says "The recommended load/decoupling capacitance is a 10uF  tantalum or a 50uF aluminum. These values will assure stability for the majority of applications"

It is well known that inadequate load capacitors can cause instability and failed regulation, so therefore please add a 47uf electrolyte (a 100uf electrolyte capacitor would be very safe) and your problems should disappear.
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please add a 47uf electrolyte (a 100uf electrolyte capacitor would be very safe) and your problems should disappear.

Ok, shall do. I learnt my lesson the first time not to ignore capacitors entirely (the 6V was putting out 3V, but had no capacitor), so now I know I should read the datasheet and not only add a capacitor, but add one with the right capacitance. Thanks. smiley
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Quote
And the datasheet (warning: PDF)
Shouldn't the warning be about the language?

Are you sure that you have a 3.3 volt version of the regulator and not an adjustable version?


Don
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Ok, just a followup:

I got tired waiting for the 100uF capacitors I ordered so took apart an old mouse and used the 220uF aluminium one inside it as a filter capacitor.

IT WORKED! This has really driven home the importance of having not just any old capacitor, but one that is actually suitable for the regulator being used. For anyone who's come here somehow through google, check your datasheets!

Thanks all for the prompt and polite help, this is why I love the arduino community.
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